Field of the Invention
This invention relates generally to active microphonic noise cancellation.
Brief Description of the Related Art
Development of digital electronics, system identification and adaptive filtering techniques are allowing new approaches to improve the performance of radiation detectors. Powerful and affordable field programmable gate arrays (FPGAs), as well as high rate and resolution analog-to-digital converters are allowing cost effective digital processing algorithms specially designed for nuclear instrumentation. In this paper we are proposing an approach to reduce the microphonic noise and improve energy, timing, position and tracking resolution of radiation detectors.
Microphonic noise in radiation detectors is associated with mechanical disturbances. These disturbances interact with the structure of the detector enclosure and its components, exciting mechanical vibrations. In one of the processes responsible for this noise, vibrations in the structure change capacitances inside the detector enclosure, injecting charge into the detector itself or its cables. This charge adds to the detector output and is measured as microphonic noise, degrading its performance.
There are several sources for these mechanical disturbances. We will now describe a few examples. Vacuum pumps can be installed in the proximity of the detector, causing vibrations that are transmitted to the detector enclosure. In general, high resolution experiments require detectors operating at cryogenic temperatures to reduce leakage current. These temperatures can be achieved using piston driven cryocoolers mounted as part of the detector assembly (e.g., for portable radiation detector systems). The electrical motor and piston of the cryocooler generate vibrations that propagate to the detector enclosure. Other systems use cryostats with detectors cooled by Dewars mounted as part of the cryostat and with an external source of liquid nitrogen. The nitrogen “bubbling” inside the Dewar may cause vibrations. Even audible noise in the environment close to the detector may interfere with the detector enclosure. Therefore, microphonic noise is difficult to control and mitigate.
Conventional filtering in nuclear spectroscopy is based on pulse shaping, substantially reducing the impact of microphonic noise (as well as other noise sources). However, if the mechanical resonant frequencies have components similar to the actual frequencies of the detector pulse, the shaper may allow the noise to propagate to the multichannel analyzer, degrading the energy resolution.
The impact of microphonic noise can be more severe in multisegmented detectors, where the actual shape and amplitude of the detector pulse waveforms are used to estimate the interaction point and tracking of the gamma rays within the detector volume. Since information is contained on the shape and amplitude of the pulses themselves, there are fewer opportunities to filter the noise in these signals because traditional shaper filters cannot be used.
The literature describes several approaches to reduce microphonic noise in high energy resolution radiation detectors. They are used mostly in nuclear spectroscopy. Various references propose an adaptive filter that uses a priori information about the exact form of the pulse signal after the charge sensitive amplifier, adapting a shaper to attenuate the microphonic noise deviating from this form. Other solutions deal with low-frequency periodic noise induced by mechanically cooling devices (e.g., cryocoolers). For example, variations on the shaper amplifier are used to minimize these contributions and another reference proposes a counterweight to mechanically minimize these system disturbances. However, observe that these approaches focus on improving the energy resolution by implementing enhanced shaper amplifiers, but they do not address microphonic noise in the detector output waveforms, which impact timing, position and tracking resolution.